The disclosure relates to a positioning device or valve assembly for a process plant, such as a petrochemical plant, a brewery, a nuclear plant or the like.
The positioning device comprises a valve such as a control valve with a valve housing, a valve seat and a valve member, a top, such as a yoke, and a bonnet or a drive-rod guide or drive-shaft guide. The top is designed to be screwed into a lead-through opening or guide opening in the valve housing, in order to be used as a supporting structure for connecting an actuator housing. The positioning device does not necessarily have to be equipped with an actuator, such as a pneumatic actuator, fastened to the top. The control valve can also be installed later. Usually, however, the positioning device is a unit made up of control valve, top and actuator.
An example for a positioning device is known from U.S. Pat. No. 6,905,108 B2, in which within a support structure, which is produced in one piece, the top realizes both a yoke function and also a drive-rod guiding function. A sealing arrangement, such as a sealing package, is accommodated in a drive-rod duct of the top, in order to seal the control valve interior in the region of the duct for the actuator rod or the actuator shaft of the actuator.
A proven positioning device is known from DE 10 2010 025 635 B4, in which the top accommodates the sealing package and also compression springs for axially prestressing the sealing package. The top is screwed into the lead-through opening of the valve housing. The top is used to guide the drive rod of the actuator for actuating the valve member with respect to the valve seat.
When screwing in the top including the yoke, assembly difficulties arose, especially in the case of inexperienced operating personnel. Due to the lack of rotational symmetry of the top, it is difficult in the case of the inherently proven screwing of the top, to adjust a certain relative position of the top with respect to the valve housing or the actuator housing, without either over-rotating the top with respect to the valve housing or imparting insufficient screwing force thereto. Owing to the rather high lever arms of the yoke structure with respect to the screwing axis, it may also come to pass that an inadvertent detaching of the top from the valve housing is caused, especially if, upon reaching the desired rotational position of the top, insufficient screwing forces are still acting between the top and the valve housing.